Patients who suffer from the pain and immobility caused by osteoarthritis and rheumatoid arthritis have an option of joint replacement surgery. Joint replacement surgery is quite common and enables many individuals to function properly when it would not be otherwise possible to do so. Artificial joints are usually comprised of metal, ceramic and/or plastic components that are fixed to existing bone.
One type of joint replacement surgery is shoulder arthroplasty. During shoulder arthroplasty, the humeral head must be resected to allow for the insertion of a humeral stem into the intramedullary canal of the humerus. The proximal end of the humerus includes the humeral head, which articulates with the glenoid cavity of the shoulder in a ball and socket fashion. The humeral head is nearly hemispherical in form.
The prostheses typically used for shoulder arthroplasty include a stem portion designed to extend into the intramedullary canal of the humerus and a head portion designed to replace the humeral head. The head portion of the prosthesis extends angularly from the stem portion. The resection of the natural humeral head must be made so that the angle of the cut corresponds to the angle between the stem and head portions of the prosthesis. In addition, the rotation of the cut varies to adjust to bone wear or capsulor looseness.
There are eight essential variables relating to humeral arthroplasty. These include: the diameter of curvature of the prosthesis; the percentage of the sphere with this diameter that will be used as prosthetic articular surface; the superior/inferior position of the articular surface relative to the humerus; the anterior/posterior position of the articular surface relative to the humerus; the medial/lateral articular aspect of the articular surface with respect to the humerus; the anterior/posterior angulation (flexion/extension) of the articular surface relative to the prosthesis; the medial/lateral angulation (varus/valgus) of the prosthesis relative to the humerus; and, the rotational alignment of the prosthetic head with respect to the humeral axis. The goal of prosthetic arthroplasty is to duplicate the normal orientation of the humeral articular surface as well as its diameter of curvature and percentage of the sphere.
Many orthopaedic companies currently provide anatomically variable prosthesis with stems that facilitate adjusting the prosthesis to more accurately reflect the anatomy of the individual. For anatomically variable prostheses, most surgical techniques call for a “freehand” cut of the humeral head. Others have rudimentary guides that facilitate a planar cut but only allow for anterior/posterior (version) or medial/lateral adjustment of the cutting plane.
When the humeral head resection is made free hand, the elbow of the patient is flexed to 90° with the patient's forearm aimed at the midline of the operating surgeon's trunk. The humerus is externally rotated to provide the recommended degree of retrotorsion in relation to the axis of elbow motion. The resection is directed away from the surgeon, allowing the surgeon to reproduce the desired retrotorsion in the bone cut. A trial prosthesis may also be placed along the proximal humeral shaft as a guide for the proper inclination of the resection. The possibility for error exists with this free hand approach. Inaccurate resection can result in an ill-fitting prosthesis which may cause complications for the patient and may eventually require replacement of the prosthetic device.
Also, when implanting a proximal humeral resurfacing implant with an extended articulation surface, removal of part or all of the humeral greater tubercle is needed. This removal should allow for proper fitting and fixation of the implant and the extended articulation surface to the resurfaced humeral head and requires cutting in two planes. However, current cutting guides only allow for the cutting of the humeral greater tubercle in one plane at a time. Therefore, the surgeon would need to perform at least two cutting steps (and possibly use two different tools) to properly prepare the humerus. There is a need for a cutting guide that allows for a surgeon to be able to properly remove the humeral greater tubercle in a single step.